Using microwave technique for information transmission to and from transponders has been known for a long time and is described in inter alia U.S. Pat. Nos. 4,242,661 "Device for registration of objects" and 4,390,880 "Radio communication system and transmitter and receiver equipment therefor".
It is further known from U.S. Pat. No. 4,673,936 to integrate a microwave unit with a wrist watch, but that invention is intended as an emergency transmitter and comprises an active frequency sweeping microwave oscillator built into the watch and intended to be detected by a reconnaissance radar onboard a ship, thereby differing from the present invention in which no microwave oscillator in included, but in which information from the transponder is fetched through passive modulation and reflection of an illuminating microwave signal. In addition, the mentioned device uses an entirely different antenna technique than the one presented here.
In many applications simultaneous demands are placed on good range, directionality and compact dimensions in the transponder, while, at the same time, the transmission frequency, which determines the size of the antenna, is predetermined for technical reasons or due to authority regulations.
It is general for all antennae that their dimension in otherwise static conditions are in inverse proportion to their operational frequency. Thus, an antenna at e.g. 5 GHz has half the extension of a 2.5 GHz antenna.
Furthermore, the capability of the antenna to receive microwave radiation is directly proportional to its effective antenna area, and since a lower frequency antenna due to its larger physical dimensions has a larger antenna area (square meter), it is capable of detecting more power (Watt) if it is in a microwave field of a certain power density (Watt per square meter) than would a higher frequency antenna with the same direction characteristics.
A high-frequency antenna can per se be given an increased effective antenna area by grouping several smaller antenna means side by side, but then there is the disadvantage of greater complexity and in that its directionality increases unfavourably, because the transponder must be directed carefully at the write/read unit in order to obtain a good range.
A transponder with a large effective antenna area and few antenna means in proportion to its total physical area therefore gives great advantages in applications with simultaneously high demands on compactness, range and freedom of orientation at a given radiation level.
Usual transponder frequencies are 2.45 GHz and 5.72 GHz, as there are internationally provided frequency bands which are commonly used for identification systems. Also 24 GHz has been provided internationally but has not yet come into any practical use, foremost due to high costs and low range performance. 2.45 GHz is also used by microwave ovens and through their widely spread usage, among other things, it is anticipated that this frequency will remain publicly available in the foreseeable future.
The mentioned frequencies are sufficiently low to enable the use of inexpensive silicon semiconductors and microstrip solutions in the microwave circuits, at the same time as the requirements for mechanic precision in impedance determining microstrip elements are moderate. The frequency is, however, sufficiently high for printed microstrip antennae to be used, which makes thin and easily manufactured antenna designs possible.
A well designed transponder with microstrip antenna has usually a radiation diagram that has its principal direction in the normal plane of the antenna layer, outwards from the antenna layer, while its sensitivity in the opposite direction is considerably depressed provided that the transponder antenna has a ground plane whose extension is considerably larger than that of the antenna layer. In practical cases, e.g. in so-called patch antennae for identification tags, the extension of the ground plane is usually chosen to approximately double the size of the antenna layer in order not to cause problems with undesirable back lobe effects.
A problem will then be that the microstrip antennae in transponder applications with directionality demands require large mounting space, whereby the applications are limited to such cases where there is space for this ground plane. Often other transponder components as well, such as e.g. power supplying means, microwave circuits, diodes, integrated circuits and passive components, require a considerable mounting area. If the ground plane could be diminished the demands would be accentuated on alternative and more space saving modes of construction for the mentioned components.
A ground plane diameter smaller than about twice the diameter of the antenna layer disturbs the antenna diagram so that its back lobe increases gradually with decreasing ground plane. If, therefore, the size of the antenna layer has been chosen to about 2 cm, which is typical at 2.45 GHz frequency, and ceramic substrate with a relative dielectricity constant of about 10, the ground plane of the antenna should have at least 4 cm diameter in order to obtain a controlled directionality without back lobe.
When there is a backward lobe formed by a limited ground plane, a signal between the communication unit and the transponder antenna can also go via reflections in objects located behind or diagonally behind the antenna layer, which causes interference between direct and reflected signal with the risk of signal drop out in positions where reflected and direct signals vectorially cancel each other.
A problem has been that with known technique it has not been possible to integrate an inexpensive microwave transponder in compact applications, e.g. into wrist watches, within usual dimensions and weight, but one has been confined to systems operating with inductive technique where a coil in the watch is used as an antenna and where the range becomes extremely limited. With inductive technique it is neither possible to achieve the directionality that is possible with microwave technique, and it is difficult to achieve a high data transmission speed.
A microwave based product would otherwise be capable of solving many problems with electronic passage control, electronic payment etc. Applied to wrist watches the technique would provide for directionality, good range, high data transmission speed and power saving, whereby very available and safe systems could be built. Applied to 5.7 GHz frequency range and a ceramic substrate the diameter of the transponder will only be about one centimeter, which can be included without difficulty in a small ladies' watch.
In addition, as the cost for a microwave based write/read unit is low, the applications could embrace most of the applications that are currently covered by magnetic cards, memory cards, microprocessor cards, various key systems and various ticket systems. In an embodiment with particularly high sensitivity to a received microwave signal the device could furthermore be used as a pager.
The most compact microwave based transponder systems that currently exist are transponders in credit card format, but these are usually carried in a wallet or pocket and thereby require to be taken out before access can occur. Furthermore, the risk of theft is obvious, as is the problem of losing the transponder.
For applications in material flow automation and traffic information applications antennae have been used hitherto with a more conventional design, with the antenna layer facing the direction of radiation and with a quite large ground plane. Although this has admitted great freedom in installing the transponders on an optional mounting area, e.g. in a car window, on a plastic palette etc. without any problem of multiple-way extension from appearing back lobe radiation, there are nevertheless many applications where it is always known that the transponder is going to be mounted on a metal plane, while on the same time there is a need for the transponder to be small. Some examples are given of identifying gas bottles and beer-cans, which must be identified on refilling and distribution.
On these products there is extremely little space available for the identity tags/escort memories, and in addition they have to be protected from the very rough treatment and chemical effects that occur when the products are handled, when at the same time there are quite high demands on range and safe identification also in dirty environments. A small transponder according to the invention would here make possible an automatic information handling around the material flow.
Another example is given on automatic reading of the mileage of a rental car. Today, this must be effected manually, while with a product according to the invention, integrated with an electronic mileage counter in the car wheel hub, with microwave technique it would be possible to perform the reading automatically. The space available at the wheel hub is very much limited, but could include a transponder for automatic remote reading according to the invention.
Although choosing higher microwave frequencies would make smaller dimensions in the transponder antenna possible, other limitations would come instead, such as size and cost of other components, difficulties with international frequency permissions and increased sensitivity to environmental factors such as waterfilm and dirt on the transponder.